compiler, runtime: simplify select and channel operations

    
    In preparation for upgrading libgo to the 1.9 release, this
    approximately incorporates https://golang.org/cl/37661 and
    https://golang.org/cl/38351.
    
    CL 37661 changed the gc compiler such that the select statement simply
    returns an integer which is then used as the argument for a switch.
    Since gccgo already worked that way, this just adjusts the switch code
    to look like the gc switch code by removing the explicit case index
    expression and calculating it from the order of calls to selectsend,
    selectrecv, and selectdefault.
    
    CL 38351 simplifies the channel code by not passing the unused channel
    type descriptor pointer.
    
    Reviewed-on: https://go-review.googlesource.com/62730

From-SVN: r252749